DESCRIPTION: The applicant proposes to examine the effects of space flight and microgravity on the cells of the developing cerebral cortex. These studies will examine three cellular processes: 1) cell proliferation, 2) neuronal migration, and 3) neuronal differentiation, including dendrite/axon orientation and acquisition of appropriate neuronal circuitry. Two markers of cell proliferation (bromodeoxyuridine and tritiated thymidine) will be administered to pregnant rats on selected days during the development of the cerebral cortex. Short term analyses (sacrifice fetuses 2.5 hrs post-injection) will examine the responses of cell proliferation to space flight by examining changes in the number of proliferating cells or the length of the cell cycle. Intermediate-term effects will be assessed by sacrificing the fetal animals 1-3 days post-injection. The migratory fates of cells within the developing cerebral cortex will be examined. Long-term effects will be assessed by sacrificing animals that were treated as fetuses at different ages through 30 days after birth. The final position of cells born at particular ages will analyzed.